1. Field of the Invention
The present invention relates to a system for measuring a distance to an object of distance measurement and more particularly to an active distance measurement system favorably applied to various types of cameras.
2. Related Background Art
Such an active distance measurement system applied to cameras generally includes an infrared-emitting diode (IRED) for emitting a beam of light toward an object of distance measurement, and a position sensitive detector (PSD) for receiving the emitted and reflected light beam. The signal outputted from the PSD is processed in signal processing and arithmetic circuits to be inputted therefrom as distance information into a central processing unit (CPU), by which a distance to the distance measurement object is determined. Because an error may occur in distance measured based on one-shot light emission alone, multi-shot light emissions are generally employed to obtain multiple pieces of distance information. The obtained information is typically integrated and averaged in an integrating circuit.
FIG. 1 shows a circuit diagram illustrating a configuration of the integrating circuit in the above distance measurement system. The integrating circuit generally shown at 16 comprises a switch 1, an integrating capacitor 2, a switch 3, a constant current source 4, an operational amplifier 5, a switch 6, a reference power source 7, a comparator 8, and a switch 9. The inverting terminal (xe2x88x92) of the operational amplifier 5 is connected through the switch 1 to the output terminal of an arithmetic circuit (not shown) and grounded through the integrating capacitor 2. Also, the amplifier inverting terminal (xe2x88x92) is connected through the switch 3 to the constant current source 4, through the switch 9 to the terminal of a power supply voltage VCC, and through the switch 6 to the output terminal of the operational amplifier 5. The non-inverting terminal (+) of the operational amplifier 5 is connected to the reference power supply 7, which provides a reference voltage VREF. The comparator 8 is connected to the junction between the inverting terminal (xe2x88x92) of the operational amplifier 5 and the integrating capacitor 2 and compares the potential of the junction and the reference voltage VREF to find out which is higher. The comparator 8 outputs a signal corresponding to the comparison results. A not shown central processing unit (CPU) receives the signal outputted from the comparator 8 and controls the on-off operation of the switches 1, 3, 6 and 9.
FIG. 2 shows timing chart 40 explaining changes in voltage level of the integrating capacitor 2 with time in the prior distance measurement system. In the integrating circuit 16, when a release button is half- or partially-depressed after the main power source is turned on, the switch 9 is put in an xe2x80x9conxe2x80x9d state only for a constant period of time under the control of the aforementioned CPU to cause the integrating capacitor 2 to be excessively charged to the power source voltage VCC. Even after the switch 9 is turned to the xe2x80x9coffxe2x80x9d state, the switch 6 is maintained in an xe2x80x9conxe2x80x9d state for another period of time so that the integrating capacitor 2 is charged up to the reference voltage VREF provided by the reference power source 7. After the charging up, the switch 6 is turned to the xe2x80x9coffxe2x80x9d state.
Then, the IRED emits pulsed infrared light and the switch 1 is turned to the xe2x80x9conxe2x80x9d state for each constant time during the emission duration. As a result, the integrating capacitor 2 accepts from the arithmetic circuit an output signal thereof as a negative voltage, which corresponds to each emitted infrared light pulse. Thus, as shown in the timing chart 40 of FIG. 2, the voltage of the integrating capacitor 2 decrementally changes step by step in value corresponding to a distance. This is called a xe2x80x9cfirst integrationxe2x80x9d.
After the predetermined number (e.g., 256) of negative voltage inputs (discharges) into the integrating capacitor 2 are completed, the switch 3 is turned to. the xe2x80x9conxe2x80x9d state by the control signal from the CPU, whereby the integrating capacitor 2 is charged at a constant rate defined by the rating of the constant current source 4. This is called a xe2x80x9csecond integrationxe2x80x9d.
During the period of the second integration, the comparator 8 always compares the voltage level of the integrating capacitor 2 and the reference voltage VREF to find out which is higher and when determined that they are coincident with each other, causes the switch 3 to be turned to the xe2x80x9coffxe2x80x9d state. This causes the charging of the integrating capacitor 2 to be stopped and the CPU to commence determining a time required to perform the second integration. As the charging by the constant current source 4 is uniform in rate, the sum of the signal voltages inputted in the integrating capacitor 2 during one distance measurement, that is, the distance to the object of the distance measurement can be determined from the aforementioned time required to perform the second integration. In the subsequent distance measurement, as the required charging of the integrating capacitor 2 has been realized by the constant current source 4, the switch 3 may be retained in the open state, unless the constant current source 4 is provided in use for a long time.
In the active distance measurement system as explained above, it is desired to use a low-cost ceramic condenser as an integrating capacitor for the integrating circuit 16 because of the requirements for decrease of production cost. However, the ceramic condenser encounters the problem of a drop in charged voltage due to dielectric absorption. That is, the capacitor 2 forms an equivalent circuit shown in FIG. 3 immediately after the start of the first charging. Because of this, in FIG. 3, when a switch SW is turned to xe2x80x9coffxe2x80x9d after the first charging, the voltage drop due to a resistance element Rx is observed. Such a phenomenon is called xe2x80x9cdielectric absorptionxe2x80x9d, which may constitute one of the factors causing an error in distance measurements.
Thus, in the aforementioned active distance measurement system, the CPU instructs at the start of the distance measurement that the switch 9 is turned to xe2x80x9conxe2x80x9d for the predetermined period of time to overcharge the integrating capacitor 2 to the voltage level higher than the reference voltage so that the voltage drop due to the dielectric absorption forcedly occurs in the integrating capacitor 2. Because the system operates in such a manner, no voltage drop occurs in the integrating capacitor 2 due to dielectric absorption during the distance measurement, thus preventing the occurrence of the distance measurement error with the result that the dielectric absorption problem can be solved.
However, the above distance measurement system must accomplish, under the instruction from the CPU, not only the operations to turn the switch 6 to the xe2x80x9conxe2x80x9d state to charge up the integrating capacitor 2 to the reference voltage VREF, to control the on-off action of the switch 1 to perform the first integration, and to turn the switch 3 to the xe2x80x9conxe2x80x9d state to perform the second integration, but also the operation to turn the switch 9 to the xe2x80x9conxe2x80x9d state to overcharge the integrating capacitor 2. As a result, complicated wiring is required to transmit the control signals representative of the instruction from the CPU. Furthermore, when the aforementioned signal processing circuit, arithmetic circuit and integrating circuit are consolidated into an integrated circuit, terminals in the integrated circuit increase in number.
The present invention has been made in order to overcome the above drawbacks and has for its object to provide a distance measurement system, which can measure a distance with high accuracy and can decrease the numbers of terminals and lines.
With the above object in view, the invention provides a distance measurement system comprising: (1) means for emitting a beam of light toward an object of distance measurement; (2) means including a position sensitive detector for receiving the beam of light emitted toward and reflected from the object at a receiving position on said position sensitive detector corresponding to a distance to the object, said light receiving means outputting a signal corresponding to the receiving position; (3) arithmetic means for carrying out a calculation based on the signal outputted from said light receiving means to output a signal corresponding to the distance to the object; (4) means including an integrating capacitor charged to a reference voltage for integrating the signal outputted from said arithmetic means over time by charging or discharging said integrating capacitor by an amount corresponding to the signal outputted from said arithmetic means, said integrating means outputting a signal corresponding to the results of the integration; (5) means for determining the distance to the object based on the signal outputted from said integrating means; and (6) charging means detecting whether a supply of power is started for overcharging said integrating capacitor of said integrating means to a voltage level higher than the reference voltage for a constant period of time after the start of the power supply.
With the arrangement of this distance measurement system, by the charging means detecting the start of the supply of power, the integrating capacitor of the integrating means is overcharged for the predetermined period of time after the detection to the voltage level higher than the reference voltage. During the subsequent distance measurement, the beam of light is outputted from the emitting means toward the object of distance measurement and reflected by the latter. The reflected light is received by the receiving means at the receiving position on the position sensitive detector corresponding to the distance to the object, and the light receiving means outputs the signal corresponding to the receiving position. The arithmetic means carries out the calculation based on the signal outputted from the light receiving means and outputs the signal corresponding to the distance to the object. The signal outputted from the arithmetic means is inputted into the integrating means and the integrating capacitor thereof charged to the reference voltage integrates the signal outputted from the arithmetic means over time by discharging from the integrating capacitor by an amount corresponding to the signal outputted from the arithmetic means. The integrating means outputs the signal corresponding to the results of the integration. The detecting means determines the distance to the object based on the signal outputted from the integrating means.
According to the preferred embodiment of the present invention, the distance measurement system further comprises control means for setting the system in a standby mode by suspending the supply of power thereto when no operation is carried out during a predetermined period of time and for releasing the standby mode to restart the supply of power when any operation is carried out during the period of the standby mode. The charging means detects whether the standby mode is released, and it overcharges the integrating capacitor of the integrating means to a voltage level higher than the reference voltage for a constant period of time after the release of the standby mode. In this embodiment, the integrating capacitor of the integrating means is overcharged, by the charging means detected the release of the standby mode, to the voltage level higher than the reference voltage for the constant period of time after the standby mode release. The subsequent distance measurement is performed in the similar manner.
As described in the foregoing, when the charging means detects the start of the power supply or the release of the standby mode, the integrating capacitor of the integrating means is overcharged by the charging means for the predetermined period of time after the detection to the voltage level higher than the reference voltage. During the overcharging, the voltage drop occurs due to the dielectric absorption in the integrating capacitor, resulting in high accuracy of distance measurement accomplished subsequent to the overcharging. Furthermore, as no additional signals are required to command the overcharging of the integrating capacitor, terminals and wiring may be simplified when the associated elements are incorporated into an integrated circuit.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.